DS2505 Maxim, DS2505 Datasheet - Page 5

• Manufacturer Part Number: DS2505
• Manufacturer: Maxim

DS2505
EPROM STATUS BYTES
In addition to the 16384 bits of data memory the DS2505 provides 704 bits of Status Memory accessible
with separate commands.
The EPROM Status Bytes can be read or programmed to indicate various conditions to the software
interrogating the DS2505. The first 8 bytes of the EPROM Status Memory (addresses 000 to 007H)
contain the Write Protect Page bits which inhibit programming of the corresponding page in the 16384-bit
main memory area if the appropriate write protection bit is programmed.
Once a bit has been
programmed in the Write Protect Page section of the Status Memory, the entire 32-byte page that
corresponds to that bit can no longer be altered but may still be read.
The next 8 bytes of the EPROM Status Memory (addresses 020 to 027H) contain the Write Protect bits
which inhibit altering the Page Address Redirection Byte corresponding to each page in the 16384-bit
main memory area.
The following 8 bytes within the EPROM Status Memory (addresses 040 to 047H) are reserved for use
by the iButton operating software TMEX. Their purpose is to indicate which memory pages are already
in use. Originally, all of these bits are unprogrammed, indicating that the device does not store any data.
As soon as data is written to any page of the device under control of TMEX, the bit inside this bitmap
corresponding to that page will be programmed to 0, marking this page as used. These bits are
application flags only and have no impact on the internal logic of the DS2505.
The next 64 bytes of the EPROM Status Memory (addresses 100H to 13FH) contain the Page Address
Redirection Bytes which indicate if one or more of the pages of data in the 16384-bit EPROM section
have been invalidated by software and redirected to the page address contained in the appropriate
redirection byte. The hardware of the DS2505 makes no decisions based on the contents of the Page
Address Redirection Bytes. These additional bytes of Status EPROM allow for the redirection of an
entire page to another page address, indicating that the data in the original page is no longer considered
relevant or valid. With EPROM technology, bits within a page can be changed from a logical 1 to a
logical 0 by programming, but cannot be changed back. Therefore, it is not possible to simply rewrite a
page if the data requires changing or updating, but with space permitting, an entire page of data can be
redirected to another page within the DS2505 by writing the one’s complement of the new page address
into the Page Address Redirection Byte that corresponds to the original (replaced) page.
This architecture allows the user’s software to make a “data patch” to the EPROM by indicating that a
particular page or pages should be replaced with those indicated in the Page Address Redirection Bytes.
To leave an authentic audit trail of data patches, it is recommended to also program the write protect bit
of the Page Address Redirection Byte, after the page redirection is programmed. Without this protection,
it is still possible to modify the Page Address Redirection Byte, making it point to a different memory
page than the true one.
If a Page Address Redirection Byte has a FFH value, the data in the main memory that corresponds to
that page is valid. If a Page Address Redirection Byte has some other hex value, the data in the page
corresponding to that redirection byte is invalid, and the valid data can now be found at the one’s
complement of the page address indicated by the hex value stored in the associated Page Address
Redirection Byte. A value of FDH in the redirection byte for page 1, for example, would indicate that the
updated data is now in page 2. The details for reading and programming the EPROM status memory
portion of the DS2505 are given in the Memory Function Commands section.
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